Abstract:

Methods of implanting dopants into a silicon substrate using a
predeposited sacrificial material layer with a defined thickness that is
removed by sputtering effect is provided.

Claims:

1. A method of implanting ions in a silicon substrate, comprising:forming
a sacrificial material layer on the silicon substrate to a defined
thickness prior to implanting said ions; andimplanting a defined dosage
of the ions into the silicon substrate while decreasing the thickness of
the sacrificial material layer such that substantially no sacrificial
material remains on the silicon substrate upon completion of the
implanting of said dosage, wherein the defined thickness of the
sacrificial material layer is effective to maintain said sacrificial
material layer over the silicon substrate to substantially eliminate
sputtering of the silicon surface during the implanting of the defined
ion dosage.

2. The method of claim 1, further comprising, prior to forming the
sacrificial material layer, determining the defined thickness of the
sacrificial material layer based on sputtering rate of the sacrificial
material layer and the defined dosage of implanted ions.

3. The method of claim 1, wherein the sacrificial material layer has a
sputtering rate lower than the silicon substrate.

5. The method of claim 4, wherein the sacrificial material layer comprises
a carbon material selected from the group consisting of amorphous carbon
and silicon carbide.

6. The method of claim 4, wherein a p-type dopant is implanted.

7. The method of claim 6, wherein boron ions are implanted, and the
sacrificial material layer comprises a boron material selected from the
group consisting of boron and boron carbide.

8. The method of claim 4, wherein an n-type dopant is implanted.

9. The method of claim 8, wherein phosphorus or arsenic ions are
implanted, and the sacrificial material layer comprises a carbon
material.

10. A method of implanting ions in a silicon substrate, comprising:forming
a sacrificial material layer on the silicon substrate to a defined
thickness prior to implanting said ions; andimplanting a defined dosage
of the ions into the silicon substrate while decreasing the thickness of
the sacrificial material layer with substantially no sputtering of the
silicon surface, wherein substantially no sacrificial material remains on
the silicon substrate upon completion of the implanting of said dosage.

11. A method of implanting ions in a silicon substrate, comprising:prior
to implanting said ions, forming a sacrificial material layer on the
silicon substrate to a defined thickness; andimplanting a defined dosage
of the ions into the silicon substrate while sputter removing the
sacrificial material layer, said defined thickness effective to maintain
the sacrificial material layer over the silicon substrate to the
completion of implanting said defined ion dosage with substantially no
sputtering of the silicon substrate, whereupon the sacrificial material
layer is substantially removed from the silicon substrate by said
sputtering.

12. A method of implanting ions in a silicon substrate, comprising:prior
to implanting said ions, forming a sacrificial material layer on the
silicon substrate to a thickness based on sputtering yield of the
sacrificial material layer and a defined dosage of the implanted ions,
the sacrificial material layer having a sputtering yield lower than the
silicon substrate; andimplanting said defined dosage of the ions into the
silicon substrate while sputter removing the sacrificial material layer,
said thickness of the sacrificial material layer effective to maintain
the sacrificial material layer over the silicon substrate to the
completion of implanting said defined ion dosage with substantially no
sputtering of the silicon substrate, whereupon the sacrificial material
layer is substantially removed from the silicon substrate by said
sputtering.

13. A method of implanting a p-type dopant in a silicon substrate,
comprising:prior to implanting said dopant, vapor depositing a layer of a
sacrificial material on the silicon substrate to a defined thickness, the
sacrificial material selected from the group consisting of carbon,
amorphous carbon, silicon carbide, boron, and boron carbide;
andimplanting a defined dosage of the p-type dopant into the silicon
substrate while sputter removing the sacrificial material layer, said
defined thickness effective to maintain the sacrificial material layer
over the silicon substrate to the completion of implanting said defined
ion dosage with substantially no sputtering of the silicon substrate,
whereupon the sacrificial material layer is substantially removed from
the silicon substrate by said sputtering.

14. The method of claim 13, wherein the dopant is boron.

15. A method of implanting an n-type dopant in a silicon substrate,
comprising:prior to implanting said dopant, vapor depositing a layer of a
sacrificial material on the silicon substrate to a defined thickness, the
sacrificial material selected from the group consisting of carbon,
amorphous carbon, and silicon carbide; andimplanting a defined dosage of
the n-type dopant into the silicon substrate while sputter removing the
sacrificial material layer, said defined thickness effective to maintain
the sacrificial material layer over the silicon substrate to the
completion of implanting said defined ion dosage with substantially no
sputtering of the silicon substrate, whereupon the sacrificial material
layer is substantially removed from the silicon substrate by said
sputtering.

16. The method of claim 15, wherein the dopant is phosphorus or arsenic.

17. A method of implanting ions in a silicon substrate, comprising:prior
to implanting said ions, forming a first layer of sacrificial material on
the silicon substrate to a thickness based on sputtering yield of the
sacrificial material and a first portion of a defined total dosage of the
implanted ions, the first sacrificial material layer having a sputtering
yield lower than the silicon substrate; andimplanting said first portion
of the total defined dosage of the ions into the silicon substrate to
form a partially doped silicon substrate while sputter removing the first
sacrificial material layer, said thickness of the first sacrificial
material layer effective to maintain the first sacrificial material layer
over the silicon substrate to the completion of implanting said first
portion of the total defined ion dosage with substantially no sputtering
of the silicon substrate, whereupon the first sacrificial material layer
is substantially removed from the silicon substrate by said
sputtering;forming a second layer of sacrificial material on the
partially doped silicon substrate to a thickness based on sputtering
yield of the sacrificial material and a second portion of the defined
total dosage of the implanted ions, the second sacrificial material layer
having a sputtering yield lower than the silicon substrate; andimplanting
said second portion of the total defined dosage of the ions into the
partially doped silicon substrate while sputter removing the second
sacrificial material layer, said thickness of the second sacrificial
material layer effective to maintain the second sacrificial material
layer over the silicon substrate to the completion of implanting said
second portion of the total defined ion dosage with substantially no
sputtering of the silicon substrate, whereupon the second sacrificial
material layer is substantially removed from the silicon substrate by
said sputtering.

18. A method of implanting ions in a silicon substrate,
comprising:repeatedly forming a layer of sacrificial material on the
silicon substrate and implanting a portion of a total defined dosage of
the ions into the silicon substrate to form a doped silicon substrate,
whereinprior to implanting said ions, forming the sacrificial material
layer on the silicon substrate to a thickness based on sputtering yield
of the sacrificial material and said portion of the total defined dosage
of ions to be implanted, the first sacrificial material layer having a
sputtering yield lower than the silicon substrate; andimplanting said
portion of the total defined dosage into the silicon substrate while
sputter removing said sacrificial material layer, the thickness of the
sacrificial material layer effective to maintain the sacrificial material
layer over the silicon substrate to the completion of implanting said
portion of the total defined dosage with substantially no sputtering of
the silicon substrate, whereupon the sacrificial material layer is
substantially removed from the silicon substrate by said sputtering at
the completion of said implanting.

19. A method of implanting ions in a silicon substrate, comprising:forming
a mask on the silicon substrate;patterning the mask to expose the silicon
substrate;forming a sacrificial material layer on the exposed silicon
substrate to a defined thickness; andimplanting a defined dosage of the
ions into the silicon substrate while sputter removing the sacrificial
material layer, said defined thickness effective to maintain the
sacrificial material layer over the silicon substrate to the completion
of implanting said defined ion dosage with substantially no sputtering of
the silicon substrate, whereupon the sacrificial material layer is
substantially removed from the silicon substrate by said sputtering;
andremoving the mask.

Description:

TECHNICAL FIELD

[0001]Embodiments of the invention relate to methods for doping substrates
in semiconductor constructions.

BACKGROUND OF THE INVENTION

[0002]A continuing trend in the semiconductor industry is increased
densification and miniaturization of features. In fabricating
semiconductor devices, including resistors, capacitors and transistors,
device size must continue to shrink in order to increase the performance
and lower the cost of ultra-large scale integrated (ULSI) circuits. Among
the components that continue to reduce in scale are junctions, which are
doped regions on a wafer where dopants such as boron, phosphorus and
arsenic, are implanted into a silicon substrate. The dopants impart
desired electrical properties to the wafer by allowing silicon, normally
only a semiconducting material, to conduct current. Junctions are used to
form source and drain (S/D) regions of MOS transistors. Devices now
require shallow junctions, which are formed by implanting ions to shallow
depths on the order of about 100-500 angstroms and typically about 300
angstroms or less. The formation of ultra-shallow junctions allows
smaller device dimensions and higher circuit density.

[0003]Ion implantation is replacing diffusion as the standard technique
for introducing conductivity-altering dopant materials into semiconductor
wafers in most ULSI doping processes. FIG. 1 schematically illustrates a
conventional ion beam implanter 10 for forming doped regions in a wafer
12. In a conventional beam-line type ion implantation system, a desired
dopant material is ionized in an ion source 14 to form an ion beam 16,
the ions are accelerated to a high velocity using an accelerator 18, and
the ion beam 16 is directed at the wafer 12 situated on a wafer chuck 20.
The depth to which ions are implanted in the wafer is obtained by
controlling the energy of the ions as they impinge on the wafer surface.
The beam current in implanters generally ranges between about 1 mA to 30
mA, depending on the implant species, energy and type of implanter. The
ions penetrate the surface of the wafer and are embedded into the
crystalline lattice of the semiconductor material. The number of
implanted ions per unit area, or dose (φ), is related to beam current
I (amperes), beam area A (cm2) and implant duration t (seconds), and
typically ranges from 1011-1016 ions/cm2. The implanted
substrate is subsequently annealed (e.g., at about 900-1,100° C.)
in an inert gas (e.g., N2, etc.) to activate the dopants, i.e.,
transfer the dopants from impurities to carriers in the crystal lattice.

[0004]The reduction of device dimensions, for example, the shortened
channel lengths of MOS transistors, creates a so-called short-channel
effect (SCE). To minimize the short-channel effect, an ultra-shallow
junction depth (xj) and low enough sheet resistance (Rs) are
required for the source/drain (S/D) fabrication of MOS transistors. For
example, for 45 nm technology node based on the International Technology
Roadmap on Semiconductor 2005 (ITRS2005), it is required that the
junction depth of S/D extension is shallower than 6.5 nm and the
activated Rs of S/D extension is lower than 650 Ω/sq. To meet these
requirements as device size shrinks, ultra-low energy (e.g., <1 keV
for boron) ion implantation must be used.

[0005]Conventional beam-line ion implantation offers advantages over
traditional diffusion techniques, including (1) precise control of dose
and depth profile due to decoupling of the doping and annealing
processes; (2) low temperature processing, which allows the use of
photoresist as a mask; (3) the ability to use a wide selection of masking
material (e.g., metal, polysilicon, photoresist, oxide, etc.); and (4)
less sensitivity to surface cleaning procedures. However, despite the
advantages of conventional beam-line ion implantation processes, there
are several drawbacks relating to fundamental physical limitations such
as space charge limits, intrinsic sputtering effects, and implant angle
limits for non-planar structures for low energy implants. These
limitations create problems in microelectronics manufacturing.

[0006]With conventional beam-line ion implantation processes, the ion beam
and momentum of the ions impinging on the wafer causes sputtering of the
silicon substrate during doping, resulting in the removal of doped
silicon material. The sputtering effect by energetic ion bombardment
during ion implantation both affects the structure of the devices and the
as-implanted dopant profile. Etching, including sputtering and reactive
ion etching (RIE), is known to cause retained dose saturation of the
dopant and, in turn, sheet resistance (Rs) saturation in the wafer.
The implant dose in the substrate is removed by etching so that the dose
is saturated after the removed depth equals the implant range (Rp).

[0008]It would be useful to provide a method for optimizing bean-line ion
implants that overcomes these or other problems.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009]Embodiments of the invention are described below with reference to
the following accompanying drawings, which are for illustrative purposes
only. Throughout the following views, the reference numerals will be used
in the drawings, and the same reference numerals will be used throughout
the several views and in the description to indicate same or like parts.

[0019]FIG. 16 graphically illustrates a comparison of the retained boron
(B+) ion doses (atoms/cm2) versus nominal B doses
(ions/cm2) by DIED simulations for a silicon substrate with no
sputtering, a silicon substrate with sputtering, and a silicon substrate
with sputtering of an overlying 50 Å boron (B) film (500 eV
11B+ beam-line ion implant on silicon).

[0020]FIG. 17 graphically illustrates a comparison of the retained B
profiles (B concentration in atoms/cm3 versus depth in angstroms) by
DIED simulations for a silicon substrate with no sputtering, silicon
substrate with sputtering, and silicon substrate with sputtering of an
overlying 50 Å boron (B) sacrificial film (500 eV 11B+
beam-line ion implant on silicon).

DETAILED DESCRIPTION OF THE INVENTION

[0021]The following description with reference to the drawings provides
illustrative examples of devices and methods according to embodiments of
the invention. Such description is for illustrative purposes only and not
for purposes of limiting the same.

[0022]In the context of the current application, the term "semiconductor
substrate" or "semiconductive substrate" or "semiconductive wafer
fragment" or "wafer fragment" or "wafer" will be understood to mean any
construction comprising semiconductor material, including but not limited
to bulk semiconductive materials such as a semiconductor wafer (either
alone or in assemblies comprising other materials thereon), and
semiconductive material layers (either alone or in assemblies comprising
other materials). The term "substrate" refers to any supporting structure
including, but not limited to, the semiconductive substrates, wafer
fragments or wafers described above.

[0023]Conventional processes using beam-line ion implantation for doping a
silicon substrate cause sputtering of the silicon substrate, which can
result in a loss of the implanted dopant material from the substrate
layer. Embodiments of the invention optimize ultra-low energy beam-line
ion implantation to reduce sputtering of a silicon substrate to be doped
by use of a sacrificial material film layer that is pre-deposited onto a
silicon substrate surface. Due to the low sputtering yield and optimized
thickness of the sacrificial material film, sputtering of the silicon
substrate is eliminated and the retained dopant dose and Rs
saturations are improved. The resulting doped silicon substrate has
reduced sheet resistance and improved electrical properties.

[0024]An embodiment of a method according to the invention for doping a
silicon substrate using an ultra-low energy beam-line ion implantation
process is illustrated with reference to FIGS. 3-6. FIG. 3 illustrates a
substrate fragment indicated generally with reference to numeral 22 at a
preliminary processing stage. The substrate fragment 22 in progress can
comprise a semiconductor wafer substrate or the wafer along with various
process layers formed thereon, including one or more semiconductor layers
or other formations, and active or operable portions of semiconductor
devices.

[0025]The substrate fragment 22 comprises a substrate 24 to be doped to a
conductivity type and a masking layer 26. The substrate 24 is generally a
semiconductor material such as monocrystalline, polycrystalline or
amorphous silicon. A typical thickness of the silicon substrate 24 is
about 300-800 μm. The masking layer 26, typically photoresist, is
formed over the silicon substrate 24, and as depicted, can be exposed and
developed using conventional photolithographic techniques. Other mask
materials such as silicon dioxide, silicon nitride, carbon, among others,
can also be used. The mask 26 provides a desired pattern with openings 28
that define and expose selected areas of the silicon substrate 24 to be
doped to form, for example, source/drain (S/D) regions, polysilicon gate,
etc.

[0026]In accordance with the invention, as shown in FIG. 4, a sacrificial
material layer 30 is predeposited (arrows ↓↓↓) onto
the silicon substrate 24 (and masking layer 26) prior to ion
implantation. The material layer 30 is considered to be a sacrificial
layer such that it is consumed during a subsequent ion implantation step.
The thickness (t) of the predeposited sacrificial layer 30 is designed
and optimized such that the layer 30 is progressively consumed by
sputtering effect during the ion implantation and completely (or
substantially completely) removed from the substrate 24 by the end of the
implantation process. The presence of the sacrificial layer 30
substantially eliminates sputtering of the substrate 24 during the ion
implantation process to reduce the loss of implanted dopant (through
sputtering) and provide an increased retained dopant dose in the silicon
bulk.

[0027]The material selected for the sacrificial material layer 30
possesses a lower sputtering rate or yield than the silicon substrate 24
and is compatible with the ions to be implanted and the implantation
process that is used for implanting the ion species. The sacrificial
material is selected so that byproducts resulting from sputtering of
layer 30 during the ion implantation processing do not contaminate the
substrate or form impurities that would be incorporated into the silicon
substrate 24 and adversely affect the nature and/or functioning of the
doped substrate. Other factors considered in the selection of the
material for the sacrificial layer include low particle generation, ease
of formation and processing, reasonable cost and process integration
compatibility.

[0028]The material layer is composed of the same species or dopant type
(e.g., n- or p-type) as the dopant being implanted, or other material
that is compatible with the dopant and silicon substrate, to eliminate
contamination and not adversely affect the character of the doped
substrate. For example, in embodiments in which p-type dopants (e.g.,
boron, etc.) are ion implanted in a silicon substrate 24, the sacrificial
material layer 30 can be formed from a carbon material (e.g., amorphous
carbon, silicon carbide (SiC), etc.), or from a boron (B) material (e.g.,
boron (B), boron carbide (B4C), silicon boride (SiB6), etc.)
where boron (B) is implanted. In embodiments in which n-type dopants
(e.g., phosphorus, arsenic, etc.) are ion implanted, the sacrificial
material layer 30 can be formed from a carbon material, or from a
phosphorus (P) or arsenic (As) material where P or As, respectively, are
implanted. A boron or carbon film used as a sacrificial layer produces
only about one-third of the sputtering yield of a silicon substrate. In
addition, carbon is an electrically neutral material if incorporated into
silicon, and thus does not adversely affect the doped silicon substrate.
The sacrificial material layer can be analyzed by known techniques, for
example, by transmission electron microscopy (TEM), x-ray photoelectron
spectroscopy (XPS), and x-ray diffraction (XRD).

[0029]The sacrificial material layer 30 can be formed on the silicon
substrate 24 by various processes known in the art. For example, the
sacrificial material layer can be vapor deposited in a processing chamber
by atomic layer deposition (ALD), plasma doping deposition (PLAD),
chemical vapor deposition (CVD), plasma enhanced CVD (PECVD), or other
vapor deposition process. The use of ALD or PLAD deposition methods to
form the sacrificial material layer provides acceptable controllability
and repeatability in the nanometer regime of deposition processing.

[0030]Generally, in an ALD process, one or more precursor gas source gases
are pulsed into a deposition chamber for a selected time period (pulse
duration), the gases are vaporized and chemisorb as a monolayer onto the
substrate, and a number of consecutive deposition cycles are conducted to
deposit thin layers (e.g., about 0.2-3.0 Å per cycle) until a layer
of the desired thickness is built up on the substrate. In a PLAD process,
which is conducted under deposition conditions, reaction gases can be fed
into a reactor where an energy source generates a plasma and the gas
species react and deposit as a layer onto the surface of the substrate.
In a CVD or PECVD process, a source gas or combination of gases is fed
into a reaction chamber where the gases react and thermally decompose on
a heated substrate.

[0031]In embodiments of the invention, a sacrificial material layer 30 of
boron, phosphorus or arsenic can be formed by a vapor deposition process
using a hydride gas such as diborane (B2H6), tetraborane
(B4H10), phosphine (PH3), arsine (AsH3), or others,
in an inert carrier gas (e.g., argon, helium, nitrogen). The layer 30 can
be deposited in a single step or in multiple steps to achieve a desired
thickness.

[0032]In another embodiment, boron carbide (BxC) can be deposited as
the sacrificial material layer 30 in a vapor deposition process (e.g.,
ALD, CVD, PECVD) using a boron gas precursor such as diborane
(B2H6), tetraborane (B4H10) or boron trichloride
(BCl3), and a carbon-forming precursor (e.g., CH4,
C3H8, C3H6, etc.), as described, for example, in US
2006/0001175 (Sandhu et al., Micron Technology Inc.). For example, a
boron carbide (BxC) layer can be vapor deposited from a gas mixture
of B2H6/CH4 or BCl3/CH4/H2 or
B2H6/B4H10/borane carbonyl (BH3CO).

[0033]In other embodiments, a silicon carbide (SiC) layer can be formed as
the sacrificial material layer 30 by vapor deposition using a silicon gas
precursor such as silane (SiH4), in combination with a
carbon-forming precursor, e.g., a SiH4/hydrocarbon gas mixture, as
described, for example, in US 2002/000444 (Goela et al.; CVD SiC) and US
2006/0046345 (Akram et al., Micron Technology, Inc.).

[0034]Vapor deposition processing (e.g., CVD, PECVD) can also be used to
form a sacrificial material layer 30 of an amorphous carbon (or
transparent amorphous carbon) using one or more hydrocarbon process gas
such as propylene (C3H6), methane (CH4), acetylene
(C2H2), ethylene (C2H4), ethane (C2H6),
propane (C3H8), etc., as described, for example, in U.S. Pat.
No. 7,220,683 (Yin, et al.) and US 2006/0001175 (Sandhu et al.) (Micron
Technology, Inc.).

[0035]A boron sacrificial material layer can also be formed by physical
vapor deposition (PVD) by sputtering (sputter vapor deposition) using
solid (pure) boron or a solid boron compound such as boron carbide
(B4C) as the sputter target, as described for example, in U.S. Pat.
No. 5,672,541 (Booske et al.) and US 2006/0032525 (Olsen et al). Briefly,
in a sputter vapor deposition, a silicon substrate (wafer) is inserted
into a vacuum chamber, ions are generated and directed at a sputter
target material, and the sputtered atoms are deposited as a layer on the
substrate.

[0036]In another embodiment, the sacrificial layer can be formed by
thermal spraying (e.g., plasma spraying) a material layer onto the
substrate Boron carbide, for example, can be deposited using thermal
spray techniques, as a plasma spray generated from a powdered material
(e.g., a high-purity B4C) that is heated in a high-temperature gas
stream (e.g., plasma gas) to above its melting point as described, for
example, in U.S. Pat. No. 6,808,747 (Shih et al.). The heated, high
velocity gas and entrained molten powder strike the substrate to be
coated and the molten powder solidifies on contact with the substrate to
form a coating of the powdered material.

[0037]Referring now to FIG. 5, using conventional beam-line ion
implantation (arrows ↓↓↓), the unmasked sections of
the silicon substrate 24 are then doped to a p-type or n-type
conductivity and a selected dose using a dopant species 32 that is
compatible with or of the same type species as used for the sacrificial
material layer.

[0038]The energy used is determined by the desired depth of the implant.
In some embodiments, the ion implantation is conducted at an ultra-low
energy range, e.g., to form a shallow junction, typically about 500 eV to
about 1 keV using an ultra-low energy ion implanter (sub-2 keV). The
amount of dopant ions that is implanted is effective to provide a low
sheet resistance (Rs). For example, an implant dosage of about
1e14-1 e16 ions/cm2 at a beam energy of about 0.2-2 keV is typically
used. For a shallow junction source/drain (SD) application, a dose of at
least about 1e15 ions/cm2 or higher is typical.

[0039]The profile of the implant can be predicted using a variety of
computer simulation tools that conduct ion implantation process
simulations for a semiconductor device to determine an ion implantation
profile. For example, computer simulation techniques using known software
such as SRIM and TRIM packages can be used for modeling the ion
implantation process to achieve the appropriate implant conditions and
dose amounts. For example, the depth and profile of an ion implant
species can be estimated by using SRIM2000 (Stopping Range of Ions in
Matter), a widely available simulation program that calculates the depth
and distribution of ions implanted into materials and takes into account
the density of the material being implanted and the energy and mass of
the impacting species. A SRIM simulation program can also be utilized to
simulate and calculate the sputtering rates for different ion species
with different energies on different substrates.

[0040]Sputtering of the sacrificial material layer 30 occurs as the ion
implantation proceeds, resulting in a continuous decrease in the
thickness (t) of the sacrificial material layer 30. The sacrificial layer
is predeposited on the silicon substrate 24 to a pre-designed thickness
to maintain a film over the silicon substrate until the completion of the
ion implantation. The thickness (t) of the predeposited layer 30 is
optimized and controlled based on the implantation of a defined dose so
that the layer is completely expended (used up) at the end or completion
of an implantation step to implant the defined dose, and essentially none
of the sacrificial layer remains, as illustrated in FIG. 6.

[0041]As illustrated in FIGS. 3-6, in some embodiments, the sacrificial
layer 30 is predeposited onto the silicon substrate to a calculated
thickness (t) in a single application, and the defined dosage is then ion
implanted in one step (FIG. 5), with sputtering completely removing the
layer 30 from the substrate at the end of the implantation process (FIG.
6). The optimal required thickness of the sacrificial material layer 30
for complete removal by self-sputtering by the end of the implantation
process can be calculated and determined based on factors such as the
sputtering yield data or sputtering rate of the sacrificial material, the
ion implantation species, the nominal dose amount that is applied, and
process conditions such as the implant energy.

[0042]In embodiments of the invention in which the sacrificial layer 30 is
deposited and the entire defined dosage is then ion implanted, the
thickness of the predeposited sacrificial material layer 30 is typically
about 40-60 angstroms. For example, in the use of a boron (B) sacrificial
material layer 30 for implanting boron (B) ions 32 at a low implant
energy of 500 eV, based on a sputtering rate of the boron sacrificial
layer of about 1 Å per 4e13 ions/cm2 nominal dose and a required
nominal boron dose of 2e15 ions/cm2, the deposit of an about 50
Å sacrificial boron layer will maintain a sacrificial boron film over
the silicon substrate for the duration of a beam-line ion implantation to
implant the identified dose without sputtering of the silicon substrate,
with the layer 30 being completely removed from the substrate at the end
of the implant.

[0043]In another embodiment of the invention, illustrated in FIGS. 7-10,
the ion implantation can be conducted in two or more stages to implant
portions of the total defined dose amount, with the sacrificial material
layer being predeposited before each implanting step. Again, the
thickness (t) of the sacrificial material layer is calculated according
to the dose that is implanted and the sputtering yield of the sacrificial
material. For example, for implanting an about 1 e15 atoms/cm2 dose
of boron ions 32' (of a total boron dose of 2e15 ions/cm2) in a
first step, a first sacrificial layer 30a' with a thickness (t1) of
about 25 Å can be deposited as depicted in FIG. 7, which thickness
will maintain the sacrificial layer 30a' on the silicon substrate 24' to
the completion of the first implanting step (FIG. 8). Then, prior to
implanting the remaining 1 e15 ions/cm2 boron dose, a second
sacrificial layer 30b' with a thickness (t2) of about 25 Å can
be pre-formed on the partially doped silicon substrate 24' as depicted in
FIG. 9, which will maintain a film on the silicon substrate to the
completion of the second implanting step (FIG. 10), whereupon the
sputtering effect from the ion implantation process will have completely
removed the layer 30b' from the substrate.

[0044]In embodiments of the invention in which multiple sacrificial layers
30a', 30b' are deposited and a portion of the defined dosage is then ion
implanted after each such deposition, the thicknesses (t1, t2)
of each of a first and second pre-deposited sacrificial layer 30a', 30b',
for example, are typically about 20-30 angstroms. For example, in the use
of a boron (B) sacrificial material layer 30 for implanting boron (B)
ions 32, based on an implant energy of 500 eV, a sputtering rate of the
sacrificial layer of about 1 Å per 4e13 ions/cm2 nominal dose,
and a 1 e15 ions/cm2 nominal dose for each implant step, the deposit
of an about 25 Å sacrificial boron layer for each implant step will
maintain a sacrificial boron film over the silicon substrate for the
duration of a beam-line ion implantation to implant the identified 1e15
ions/cm2 nominal dose without sputtering of the silicon substrate,
with each of the sacrificial layers 30a', 30b' being completely removed
from the substrate at the end of each of the implant steps.

[0045]Referring now to FIG. 11, after the ion implantation process is
completed, the photoresist mask layer 26 and/or other masking material
can be selectively removed (stripped) with wet chemical or dry etching or
a combination of both. Photoresist, for example, can be removed by a
standard dry etch process using an oxygen (O2) plasma ashing step.
Optionally, the silicon substrate 24 can be treated by dry or wet etching
to expose and clean the surface and remove any remaining residue of the
sacrificial material layer and/or masking material.

[0046]As depicted in FIG. 12, the doped substrate 24 can then be annealed
(arrows ↓↓↓) to activate the implanted dopant ions
32, for example, using a rapid thermal anneal process at a temperature of
about 900-1100° C., to form a shallow junction 34, for example.
The ion dose (implanted and annealed) can be measured by a SIMS
technique, and the sheet resistance (Rs) can be measured by a four
point probe technique, using conventional techniques.

[0047]The described process results in a reduction of the implanted dopant
loss caused by sputtering of a silicon substrate during beam-line ion
implantation, an increase in the retained dopant dose in the silicon
bulk, improvements in the Rs saturation, and no or minimal
structural change of the original substrate surface.

[0048]Embodiments of the invention can be used to produce shallow
junctions, polysilicon gates, etc., with required junction depth
(Xj) and sheet resistance (Rs). The doped substrate 24 can
undergo additional processing steps known in the art to fabricate desired
components. Finished semiconductor wafers can be cut into dies, which may
then be further processed into integrated circuit chips and incorporated
in electronic devices.

EXAMPLES

[0049]To illustrate the process of the invention, sputtering of various
materials situated on a silicon substrate and the ion implantation of
boron (B) and arsenic (As) ions in a silicon substrate using a
pre-deposited sacrificial material layers were investigated.

[0050]Compared to n-type impurities such as As or P, boron (B) ion species
are more critical for ultra-low energy implant applications due to the
lower mass (severe space charge effect), much lower solid solubility than
n-type impurities, segregation behavior, and the intrinsically lower
mobility of holes (thermally activated from boron impurity) than
electrons.

[0051]Table I lists the sputtering yields (at atoms/ion) of boron ions
(B+) and arsenic ions (As+) on different substrates which are
III- or IV-family impurity materials, versus the B+ and As+ ion
energy ranging from 200 ev to 2 keV.

[0053]The sputtering yield data for Tables I-II and FIGS. 13-14 was based
on an ion implantation computer simulation using SRIM2006 software (J. F.
Ziegler, http.//www.SRIM.org/). Of the listed materials, boron (B) and
carbon (C) were chosen for further study due to their lower sputtering
yield, being about one-third the sputtering yield of a silicon substrate
in an energy range of 200 eV to 2 keV.

[0054]Table II (below) lists the sputtering yield or rate data (at
Å/sec) of boron (B+) and arsenic (As+) ions implanted on
different substrates (silicon, boron, carbon) versus the implant energy
at 200 eV, 500 eV and 1 keV. The data are based on Table I and assumes
that the doping rates of the impurities at all energies (i.e., nominal
doping rate) are fixed at 4e13 cm2/sec, and the sputtering rate of
500 eV B+ ions implanted on silicon substrates is 3 Å/second.

[0055]An ion implantation simulator DIED (Dynamic Ion-implantation with
Etching and Deposition) was used to determine the final retained boron
(B) profile/dose when sputtering or deposition effects were included. A
DIED simulator is a MATLAB®-based software that iteratively computes
the retained implanted dopant profile, dose, and maximum concentration
including the etching (sputtering or RIE) and deposition effects. The
impurity profiles of DIED use Pearson-IV profile function (J. F. Ziegler,
http://www.SRIM.org/), which is a more accurate function than simple
symmetrical Gaussian function.

[0056]FIG. 15 illustrates a typical DIED simulation result assuming 500 eV
B+ ion implanted on a silicon substrate with a nominal dose of 2e15
ions/cm2 and a sputtering rate of 3 Å per 4e13 ions/cm2
nominal dose. The doping rate was 4e13 ions/cm2-sec, so that the
total implant time was 50 seconds for 2e15/cm2 nominal dose. Due to
the sputtering effect, the retained B profile becomes half Gaussian-like
function, the retained B dose is saturated at about 7e14 atoms/cm2,
and the maximum concentration is saturated at about 1.9e21 atoms/cm3

[0057]The results show that a boron (B) film can be pre-deposited as a
sacrificial film to reduce the sputtering effect to about one-third of
the sputtering yield of a silicon substrate, and improve the retained
dose loss issues caused by sputtering effects during the ion
implantation. Assuming that a boron (B) substrate has a sputtering rate
of 1 Å per 4e13 ions/cm2 nominal dose, for a nominal dose of
2e15 ions/cm2 it will take about 50 seconds to complete the implant
during which about 50 Å of the B substrate will be removed by
sputtering effect. In an embodiment of the invention, an about 50 Å
boron (B) film can be pre-deposited on a silicon wafer surface as a
sacrificial layer so that when the ion implantation is completed, the
sacrificial B film is concurrently and completely removed by the
self-sputtering action by the end of the ion implantation process. The B
dose loss from the silicon bulk by sputtering effect can thus be reduced.

[0058]FIG. 16 illustrates a comparison of the retained boron (B+) ion
dose (atoms/cm2) versus nominal B dose (ions/cm2), and FIG. 17
illustrates retained B profiles by DIED simulations for a silicon
substrate with no sputtering, silicon substrate with sputtering, a
silicon substrate with an overlying pre-deposited 50 Å boron (B)
sacrificial film, and an embodiment (discussed below) in which a
2e15/cm2 nominal dose implant is performed in two steps with each
implant step involving a pre-deposit of a 25 Å boron (B) film and
implanting half of the nominal dose (i.e., 1 e15/cm2). The results
show that when there is no sputtering effect, the retained B dose equals
the nominal B dose (FIG. 16), and the final B profile is a Pearson-IV
function with a higher maximum concentration (FIG. 17). FIG. 17 also
demonstrates a high level of agreement of boron (B) profiles between DIED
simulation and SRIM2006 simulation.

[0059]A comparison of the results of sputtering a silicon (Si) substrate
with sputtering a boron (B) sacrificial film situated on a silicon (Si)
substrate according to embodiments of the invention shows a significant
improvement on retained B dose and maximum concentration saturations. The
results indicated that the retained B dose did not reach saturation and
is about double to that of silicon (Si) substrate at a nominal dose of
2e15 ions/cm2 with a 77% retained dose increase (FIG. 16).

[0060]In another embodiment, the implant process can be divided into
multiple steps, with a sacrificial material layer deposited prior to each
implant step. For example, a 2e15 ions/cm2 dose implant can be
divided into two steps by pre-depositing a 25 Å boron (B) sacrificial
film and ion implanting one-half of the required nominal dose (i.e., 1e15
ions/cm2), with the sacrificial film being completely removed by
self-sputtering at the end of the first implant step. A second 25 Å
boron (B) sacrificial film can then be pre-deposited and the remaining
half of the required nominal dose (i.e., 1e15 ions/cm2) ion
implanted, wherein the second sacrificial film is also completely removed
by self-sputtering from the doped silicon substrate at the end of the
second implant step.

[0061]As illustrated in FIGS. 16-17, the use of a multiple step (e.g.,
two-step) implant process using multiple pre-formed 25 Å boron (B)
sacrificial layers provided a higher retained boron (B) dose and maximum
ion concentration compared to a one-step boron (B) implant process using
a single pre-formed 50 Å boron (B) sacrificial film. The results also
show that the retained B+ dose increased by 127% compared to the
B+ dose resulting from a conventional implant process of a silicon
(Si) substrate without the use of a sacrificial layer.

[0062]Table III (below) lists the simulation results of retained boron
(B+) dose, retained boron (B+) dose fraction (retained B
dose/implant nominal B dose), and process improvements (B+ dose
increase) of B+ ion implants at different energies when the exposed
substrate is silicon (Si) (conventional implant process), and by a
sputtering-less implant processing according to embodiments of the
invention using a boron (B) sacrificial film substrate over silicon by a
one-step and by a two-step deposition/implant process (based on an
implant nominal B dose of 2e15 ions/cm2). With sputtering-less
implant processing according to the invention, the retained boron doses
can be increased from about 77% to up about 244% depending upon the
implant energies and processing embodiment (i.e., 1-step or multi-step)
that are used.

[0063]Methods of the invention utilize a sacrificial material layer that
provides a lower sputtering rate than the substrate to be implanted,
resulting in an improvement (i.e., reduction) of the sputtering effect of
low energy ion implants on the dopant-implanted substrate. Sacrificial
material layers utilized according to the methods of the invention are
removed by self-sputtering during the ion implantation process, which
eliminates the need to remove the sacrificial material after the ion
implant is completed, thereby reducing the number of required processing
steps. By comparison, other materials such as oxides (e.g., SiO2)
that have a similar sputtering rate as silicon (Si) do not improve or
reduce the sputtering effect of low energy ion implants, and must also be
removed after the process has been completed.

[0064]Some embodiments of the invention, for example, utilize a boron or
carbon sacrificial layer for a boron (B+) ion implantation, and a
carbon sacrificial layer for an arsenic (As+) ion implantation.
Advantages of using carbon (C) as the sacrificial material layer (or a
boron (B) layer in the case of a boron implant) compared to other
materials include a lower sputtering yield than silicon, no contamination
of the silicon substrate, less particle generation, compatibility to the
ion implant, and ease of processing the material to form the sacrificial
layer (e.g., by ALD, PLAD, etc.).

[0065]Although specific embodiments have been illustrated and described
herein, it will be appreciated by those of ordinary skill in the art that
any arrangement which is calculated to achieve the same purpose may be
substituted for the specific embodiments shown. This application is
intended to cover any adaptations or variations that operate according to
the principles of the invention as described. Therefore, it is intended
that this invention be limited only by the claims and the equivalents
thereof. The disclosures of patents, references and publications cited in
the application are incorporated by reference herein.